1. Field of the Invention
The invention relates generally to semiconductor fabrication. In particular, the invention relates to plasma etching processes and chamber seasoning.
2. Background Art
The fabrication of modem semiconductor integrated circuits requires etching processes of carefully controlled chemistry and processing conditions in order to achieve anisotropic and selective etching of thin layers. One example involving silicon etching is illustrated in the cross-sectional view of FIG. 1. A silicon substrate 10 is covered with a thin gate oxide layer 12 having a composition of approximately SiO2 and a thickness of about 1 to 10 nm. A polysilicon layer 14 is deposited over the gate oxide layer 12. It is intended to act as a lateral conductor so it is relatively thick, for example, 120 nm or more. An anti-reflection coating (ARC) 16 is deposited over the polysilicon layer 14. Its thickness and dielectric constant are chosen to prevent spurious reflections during the photolithography, but it is typically thinner than the polysilicon layer 14. There are several types of ARC. Dielectric ARC (DARC) is an inorganic composition, most typically, of silicon oxynitride (SiON) while BARC has an organic composition. A layer of photoresist is deposited over the ARC layer 16 and photographically patterned to form a photoresist mask 18 having a width corresponding to the desired gate width.
A multi-step plasma etching process is then used for form the structure of FIG. 2. In a break through etching step, the ARC layer 16 is etched selectively to the underlying polysilicon 14. A typical breakthrough etch for silicon oxynitride is based on fluorocarbon chemistry and reactive ion etching conditions. For example, the etching gas mixture for the breakthrough etch is a combination of carbon tetrafluoride (tetrafluoromethane, CF4) and an inert gas such as argon (Ar) which is excited into a plasma at pressures of a few millitorr. The fluorocarbon plasma and in particular the fluorine radicals are effective at etching silicon oxynitride. The carbon content of the fluorocarbon forms a protective carbonaceous polymer, particularly on non-oxide portions such as the photoresist 18 and the underlying polysilicon 14. When the pedestal electrode supporting the wafer is RF biased to create a negative self-bias with respect to the adjacent plasma, the resultant reactive ion etching increases the etching rate. Further, the polymer is removed from the oxynitride exposed at the bottom of the hole being etched but continues to protect the sidewalls, thereby achieving anisotropic etching, which is desired even for the relatively thin ARC layer 16. The protective polymer produced by the fluorocarbon also deposits on the sidewalls or protective shields of the chamber. The sidewall and shields will be commonly referred to as a side portion of the reactor surrounding the processing space of the reactor. The pedestal electrode supporting the wafer being etched is adjacent to the processing space, typically on a lower side and the adjacent portions are somewhat higher and typically circularly symmetric about a central axis of the pedestal electrode.
For the organic BARC, on the other hand, a typical breakthrough recipe includes HBr and He—O2. Typically for safety reasons, oxygen is included in an O2/He mixture, also called He—O2 and which is typically a premixture of 70% He and 30% O2.
After breakthrough, a main etch directed to the polysilicon layer 14 is performed in which the etching conditions are changed to a silicon etch chemistry, such as hydrogen bromide (HBr), chlorine (Cl2), oxygen (O2), and CF4. With wafer biasing, the bromine-based chemistry anisotropically etches silicon preferentially to the underlying oxide. The fluorocarbon content provides some polymerization and keeps the chamber walls relatively free of silicon oxide. The large amount of oxygen causes the already deposited sidewall polymer to be gradually removed during the main etch. The main etch, however, is selective to the underlying oxide layer 12, thereby producing the structure illustrated in FIG. 2 even if some overetching is done.
If the wafer is electrostatically chucked to the pedestal electrode, the dechucking process often includes an unbiased or relatively soft argon plasma to neutralize charge buildup on the wafer.
The process described above is subject to some unusual behavior. The breakthrough etch for BARC is usually terminated according to an endpoint detector that spectroscopically observes the species in the plasma, and in particular whether any of the underlying silicon is being etched. In fact, the etch is typically continued 40% after endpoint. The time required to reach endpoint provides a ready measure of the BARC etch rate. The etch rate is observed to strongly depend on the previous history of the chamber. Furthermore, when electron emission from the plasma is monitored during the main etch, the emission rate is some cases jumps significantly during the main etch. It is generally considered poor practice to dramatically change the plasma state during an etching even if endpoint detection is made.
Interactions of the silicon etching plasma with the anodized aluminum wall are believed to be responsible for high aluminum levels and other defects in the etched silicon.
Plasma dry cleaning of chambers is known, typically performed after long production cycles. Also chamber seasoning is known in which a thick polymer layer is deposited on the chamber walls. See, for example, U.S. Patent Application Publication 2002/0086118 A1 to Chang et al. and U.S. Pat. No. 5,268,200 to Steger.